gdb/gdbserver: share I386_LINUX_XSAVE_XCR0_OFFSET definition

Share the definition of I386_LINUX_XSAVE_XCR0_OFFSET between GDB and
gdbserver.

This commit moves the definition into gdbsupport/x86-xstate.h, which
allows the #define to be shared.

There should be no user visible changes after this commit.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>